Demystifying Abstract Classes vs. Interfaces in Java

As an experienced Java developer, I appreciate the immense power of abstraction. But I know it can also cause confusion when starting out. Questions like:

• What‘s the difference between abstract classes and interfaces in Java?
• When should I use one or the other?
• What are the downsides to overusing abstraction?

I asked these same questions in my early coding days!

That‘s why in this comprehensive 2800+ word guide, I‘ll demystify abstract classes and interfaces in Java through:

• Simple explanations and visual examples
• Real code demonstrations of inheritance vs implementation
• Guidelines on when to use each (and when NOT to use each)
• Pros, cons and downsides to watch out for

My goal is to provide clarity on using abstraction correctly so you can write cleaner, more extensible Java code.

Let‘s start from square one…

Abstract Classes and Interfaces – A Brief Introduction

Abstraction is a core concept in object-oriented programming. It refers to simplifying complex systems by hiding unnecessary details and creating higher level groupings of related data and functions.

For example, a car is an abstraction. We group together the underlying mechanical systems, electronics, chassis, etc. into one conceptual unit – the car. This lets us work at a higher functional level rather than worrying about every small underlying detail.

In Java, abstraction lets us define generic classes and interfaces representing conceptually related items. This allows greater flexibility and re-use in software design.

Abstract classes allow partial abstraction. They contain reusable code templates others subclasses inherit. But subclasses must provide implementations missing in the abstract parent class.

Think of an abstract Vehicle class that different types inherit from:

public abstract Vehicle {

  public void startEngine() { 
    // code to start engine
  }

  public abstract void drive(); 

}

public Car extends Vehicle {

  @Override
  public void drive() {
    // code for car driving  
  }

} 

Interfaces on the other hand provide complete abstraction. They declare functionality signatures classes must implement, but nothing more:

public interface Driveable {

  public void turnLeft();

  public void turnRight();

}

public Car implements Driveable {

  @Override 
  public void turnLeft() {
    // car left turn
  }

  //...

}

The key question becomes – when should you use an abstract class vs interface? Let‘s explore that next…

Abstract Classes – Creating Inheritable Code Templates

Abstract classes create templates with partial implementations for subclasses to reuse and customize.

You define an abstract class using the abstract keyword:

public abstract class AbstractClass {

  // fields, concrete methods
  // abstract methods  
} 

Here are the key qualities of abstract classes:

• Cannot instantiate directly – Must subclass first
• Can contain both abstract and regular methods
• Subclasses inherit all fields/methods and must override abstract methods
• Used to provide code reuse through inheritance
• Partially hides implementation details from subclasses

For example:

public abstract class AbstractRepository {

  // fields 
  protected String url; 

  // concrete method
  public void connectToDataSource() {
    // connects to DB
  }  

  // abstract method  
  public abstract void queryData();

}

public MySQLRepository extends AbstractRepository {

  @Override 
  public void queryData() {
    // SQL query implementation
  }

}

The AbstractRepository defines the template for subclasses like MySQLRepository to inherit common logic – connecting to a data source. Subclasses override the queryData() method to provide custom implementations while reusing connectToDataSource().

This allows for greater code reuse across class hierarchies.

Based on my database integration experience, here are some benefits of using abstract classes:

• Promotes code reuse via inheritance – less duplicated logic
• Centralized way enforce required methods subclasses must implement
• Provides flexibility for subclasses to customize details

Potential downsides to watch out for with abstract classes:

• Too many layers of inheritance can overcomplicate designs
• Tight coupling of subclasses to parent definitions
• Changes require modifying abstract parent class

Now that we have a handle on abstract classes, let‘s contrast them with Java interfaces…

Interfaces – Defining Flexible Code Contracts

While abstract classes focus on inheritance, interfaces emphasize abstraction through flexibility and loose coupling.

You define an interface with the interface keyword:

public interface InterfaceExample {

  void methodOne();

  void methodTwo();

}

Here are the critical qualities of interfaces:

• Only contain method signatures – no method bodies
• No fields allowed – only static final fields
• Classes implement interfaces by overriding all methods
• Enable loose coupling by separating contracts from implementations
• Fully abstract underlying implementations

For example:

public interface Vehicle {

  void turnLeft(); 

  void turnRight();

}

public Car implements Vehicle {

  @Override
  public void turnLeft() {
     // code turns car left
  } 

  @Override
  public void turnRight() {
    // code for right turn 
  }

}

Based on my experience building Java systems, here are some major benefits provided by using interfaces:

• Defines capabilities classes should have without dictating details
• Enables loose coupling – swap implementations easily
• Allows multiple inheritance from interfaces
• Forces commitment to API contracts
• Separates interfaces (contracts) from implementations

Overusing interfaces can potentially lead to downsides like:

• Designs with too many small interfaces
• Complex inheritance hierarchies
• Confusing interfaces with unclear purposes

Now that we have explored abstract classes and interfaces separately, let‘s compare them directly…

Abstract Classes vs. Interfaces – A Direct Comparison

We‘ve covered a lot of ground so far. Let‘s recap and directly compare some key differences between abstract classes and interfaces in Java:

Comparison Point	Abstract Classes	Interfaces
Inheritance	Single inheritance using extends	Multiple inheritance using implements
Type definitions	Can have method stubs and state	Strict method signatures, no state
Access levels	Have access modifiers	Interface methods implicitly public
Implementations	Partial abstractions	Fully abstract
Mutability	Can have Constructors	Cannot define constructors
Composition capabilities	Only inherits from one supertype	Allows multiple interface inheritance
Coupling level	Tightly coupled to parent class	Loose coupling with implementing classes
Common uses	Skeletal code templates and partial implementations	Defining capabilities and contracts

While that may seem like a lot to digest, the key takeaway is:

• Abstract classes emphasize code inheritance between classes
• Interfaces focus on abstraction and decoupling contracts from implementations

Understanding these core differences makes choosing one over the other much clearer.

Guidelines – When to Use Each Approach

Based on the above comparisons, here are some guidelines I follow on when to use abstract classes vs interfaces:

Use Abstract Classes when:

• There is functionality you want subclasses to inherit without having to rewrite code
• You need to define skeletal methods with partial or empty implementations
• Require non-static/non-final fields to hold state for subclasses
• Want to create re-usable templates subclasses fill in

Use Interfaces when:

• Focus is purely on capabilities, not implementations
• Multiple inheritance is needed to conform to more than one contract
• Loose coupling flexibility is more important than optimizing inheritance

There is no perfect answer – it depends on intended use case and flexibility requirements.

Here is one example combining both for a banking application:

// Interface   
public interface Accountable {

  void withdrawFunds(double amount);

  void depositFunds(double amount);

}

// Abstract class
public abstract class BankAccount implements Accountable {

  // state fields 
  protected double balance;

  // constructor
  public BankAccount(double openingBalance) {
    balance = openingBalance;  
  }

  // partial deposit implementation  
  public void depositFunds(double amount) {
    balance += amount;  
  }

}  

// Concrete subclass  
public CheckingAccount extends BankAccount {

  public CheckingAccount(double openingBalance) {
    super(openingBalance);
  }

  // custom withdraw implementation
  public void withdrawFunds(double amount) { 
    balance -= amount;
  }  

}

This showcases using both an interface and abstract class to fulfill different roles:

• The Accountable interface defined core capabilities all accounts should have without specifying how to implement them
• The abstract BankAccount class offers reusable code templates subclasses inherited from

Let‘s recap everything we covered…

In Summary – Using Abstraction Effectively

We covered a lot of ground comparing abstract classes and interfaces in Java. Let‘s recap the key takeaways:

Abstract classes:

• Create inheritable class hierarchies for code reuse
• Provide skeletal implementations subclasses fill in
• Support adding fields/state

Interfaces:

• Focus purely on capabilities using method signatures
• Enable loose coupling through polymorphism
• Allow multiple inheritance

Using abstraction effectively is a core design skill in Java and OOP. By understanding when to leverage inheritance vs loose coupling, you can write more flexible software resistant to change.

Of course this just scratches the surface on mastering abstraction skills. Feel free to reach out if you have any other questions on effective use of abstract classes and interfaces!